In ferromagnetic (FM) Heusler alloys the structural transition from high temperature cubic phase to low temperature tetragonal one takes place. The experiments show that in Ni2MnX (X=In, Sn, Sb) alloys the next sequence of phase transitions occurs: paramagnetic (PM) cubic phase → FM cubic one → PM tetragonal one → FM tetragonal one [1]. This magnetic behavior can be explained by the existence in these alloys the inversion of exchange interaction [2]. In this work with the help of the Ginzburg-Landau theory the phase diagrams of Heusler alloys with inversion of exchange interaction are theoretically investigated. We used the expression for the Ginzburg-Landau functional from works [2,3]

F=αm2/2+βm2cosφ/2-γm4cosφ/4+δ1m4cos2φ/4+δ2m4/4-ω1m2(e22+ e32)/2-

ω2m2(e22+ e32)cosφ/2+a(e22+ e32)/2+be3(e32-3e22)/3+c(e22+ e32)2/4 (1)

where m is the normalized magnetization; φ is the angle between the magnetizations of two ferromagnetic sublattices; e2,3 are the linear combinations of the strain tensor; α, β, δ1, δ2, γ are the exchange interaction parameters; ω1, ω2 are the magnetoelastic constants; a, b, c are the linear combinations of the 2nd-, 3rd-, and 4th-order elastic moduli. After minimization energy with respect to the order parameters m, φ, e2,3 we constructed the phase diagrams on the (β-a) plane. The analysis shows that in general case twelve phases exist: three paramagnetic, three ferromagnetic and six antiferromagnetic phases. At certain values of functional parameters (1) we obtained the same sequence of phase transitions as in experimental work [1].

This work was supported by grants RF and CRDF Y2-P-05-19, RFBR 05-08-50341, 06-02-16266, 07-02-96029-r-ural, 06-02-39030-NNSF, 05-02-19935-YaF_a RFBR and JSPS, RF President MK-5658.2006.2 and Human Capital Foundation.